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Hey folks hopefully someone here can help me.

I've set up a bridge on my Ubuntu system to plumb into a VM I have running inside of KVM. The bridge is able to send ARP requests and replies so I have 2 way traffic but no IP traffic is able to pass.

The networking is as follows: vnet0 -> bridge -> eth1 -> VLAN with hosts I am trying to ping

Ping all fail when trying to traverse eth1. I have done TCPDUMPs on my vnet0, bridge and eth1 interfaces. I can see the problem is no IP packets leaving eth1 when sent from vnet0 while they seem to pass through the bridge interface just fine. Also of note, I am using a USB 3.0 gigabit adapter for this connection.

Troubleshooting so far:

  1. Disabled IPv6 via sysctl.conf thinking that may somehow be a problem
  2. Moved the USB adapter to a different USB port
  3. Deleted and Created bridge multiple times
  4. Reboots

Stumped. Any idea why only L2 can pass? My VM learns MAC addresses for systems through eth1 via arp but cannot sent IP packets through.

Thanks

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2 Answers 2

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After beating my head against the desk for another hour or so I figured it out. I based my /etc/network/interfaces config off of what I had working on a different machine that was running 14.04. The broken interfaces file showing only the relevant bits:

auto eth1
iface eth1 inet manual
up ifconfig eth1 up

auto E-MGMT-Bridge
iface E-MGMT-Bridge inet manual
bridge_stp off
bridge_fd 0
bridge_ports eth1

In 16.04 this breaks Layer 3 somehow while still allowing Layer 2 traffic like ARP, CDP and STP frames through. Very strange. To get it to work in 16.04 you need to remove the eth1 stanza and only define the bridge and not the physical interface it refers to like so:

auto E-MGMT-Bridge
iface E-MGMT-Bridge inet manual
bridge_stp off
bridge_fd 0
bridge_ports eth1

I had to reboot and then was able to ping from my VM across the bridge to the real network attached to my switch. Not sure if anyone else has run into this or if I am just lucky.

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    I just ran into this in 16.04.4. I am not sure why this is the problem, but defining only the bridge fixes it.
    – Doug
    May 5, 2018 at 4:15
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If you just see ARP but no "real" traffic then iptables might be your problem. Check if iptables' FORWARDING policy is set to ACCEPT. If you want to use policy DROP instead then check if forwarding rules have been added for allowing your KVM traffic.

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